I'm feeling slightly uncomforable that FigureManagerBase.init now relies on subclasses having set the window attribute already. In fact, wouldn't that break external figure managers?

Technically this only requires that set_window_title works by the time the base class init is called, but sure. We could wrap the call to set_window_title in the base class into a try... except and emit a deprecation warning if an exception is raised, to handle the case of third-party managers. A quick look shows that this would not affect either ipython's inline backend or pycharm's interagg backend, though, as they don't touch any of these.

Probably the better and more explicit way would be to have a FigureManagerBase.post_init() that set's the window title and let all backends call that.

That seems to make the coupling even worse, IMO.

Alternatively, we could pass the window to FigureManagerBase.init and set the attribute there.

I think that also breaks backcompat, and actually also break third-party backends as soon as they pass parameters up to the super class, regardless of whether this mattered to them? Also note that noninteractive backends have no notion of "window".

Try except + documenting that set_window_title() must/should* work for the base class init would technically be ok (*well it's ok, to not work because of try-except but then you loose the window title). Then again, this is such a strange documentation/requirement, that it hints at some design problem:

Either the base class should take full ownership and API responsibility for the window or none at all; i.e. either both define self.window and apply self.set_window_title() or do nothing at all.

Is there any reason not to do FigrureManagerBase:

def __init__(self, canvas, num, *, window=None):
    ...
    self.window = window
    if hasattr(self, 'set_window_title'):
        self.set_window_title(...)

if you really don't want self.window in the base class, we should introduce an intermediate WindowFigureManager class.

well it's ok, to not work because of try-except but then you loose the window title

No, because if you're on an old-style class that doesn't allow this pattern, then that class was in charge of calling set_window_title itself anyways. (The point is to later get rid of the try except and error out if a subclass doesn't allow this.)

Is there any reason not to do FigrureManagerBase: (elided)

Because set_window_title is defined in the base class, so that doesn't work anyways. Actually '.window' is not an attribute we document to be present and indeed for example FigureManagerMac has no such attribute.

rebased

I have verified that this works on the MacOSX backend on my machine, Catalina, 10.15.7. I can change the window title with, for example, fig.canvas.manager.set_window_title('testing 1234').